In recent years, as a printing plate for off-set printing, a large number of computer to plate (CTP) systems have been developed and put into practical use, which imagewise exposes directly to a planographic printing plate material employing laser light based on digital image data to record an image on the material.
Among them, in a printing plate making field in which a fairly high printing durability is required, it is known a negative type planographic printing plate material having a photosensitive layer containing a polymerizable compound (e.g., Patent document No. 1).
It is known a material having a photosensitive layer containing an iron-arene complex as a polymerization initiator so as to achieve a high sensitivity (e.g., Patent document No. 2). Further, it is also known a photosensitive layer containing a photo-reductive colorant such as a metallocene compound, a trihalogenated methyl compound, or a xanthene compound in order to achieve a high sensitivity, and a high printing durability (e.g., Patent document No. 3).
The printing plate materials having the above-described photosensitive layer exhibit relatively high sensitivity, however, they tend to show deterioration of the performance after a long-term storage and they are not fully satisfactory for the storage property.
On the other hand, for a printing plate material having a radical polymerization photosensitive layer, a preferred method for preparing a printing plate employs a thermal treatment (pre-heating) after imagewise exposure of the material before development in order to achieve a high printing durability.
However, the above-method is not fully satisfactory because it is required to control the pre-heating condition and a latitude of temperature for pre-heating is not large enough.
Further, the above negative-working photopolymerization type printing plate material has a problem in that sludge is likely to be produced in a developer, and does not have a fully satisfactory running property for a long-term development processing.
Patent document No. 1: Japanese Patent Publication Open to Public Inspection (JP-A) No. 10-104835
Patent document No. 2: JP-A NO. 1-152109
Patent document No. 3: JP-A NO. 2-127404